1. Field of the Invention
The present invention relates to a technical field of liquid crystal display device, and in particular to liquid crystal display devices employing the in-plane switching (IPS) mode and FFS mode, enabling display by applying a transverse electric field to in-plane aligned liquid crystalline molecules. The present invention also relates to an optical compensation film can contribute to improvement of contrast ratio of liquid crystal display devices employing the IPS mode or the like, and a polarizing plate using the same.
2. Related Art
As liquid crystal display device, widely used are those of so-called TN mode type, comprising a liquid crystal layer, containing a nematic liquid crystal in a twisted arrangement, placed between two orthogonal polarizing plates, and applied with an electric field in the direction normal to the substrate. This mode suffers from light leakage due to birefringence of the liquid crystalline compound when viewed at an oblique direction angle, because the liquid crystal molecules rise up from the substrate in the black state. In order to solve this problem, use of a film having a liquid crystalline compound in a hybrid alignment, aiming at optically compensating the liquid crystal cell and thereby preventing the light leakage, has been put into practical use. It has, however, been very difficult to achieve complete optical compensation of the liquid crystal cell without causing any problems, even if discotic liquid crystalline compounds are used for preparing such films, still leaving a problem of incomplete suppression of grayscale inversion in the lower portion of screen.
Aiming at solving this problem, liquid crystal display devices employing so-called IPS mode and FFS mode, applying a transverse electric field to the liquid crystal, and employing the vertical alignment (VA) mode, comprising liquid crystal of negative dielectric anisotropy vertically aligned as being partitioned by projections or slit electrodes formed in the panels, have been proposed and put into practical use. These panels have been developed not only for monitor use, but also for television use, and as a consequence, have greatly been improved in brightness. In this situation, even a small light leakage in the black state at a an oblique direction in the diagonal incident direction, which has not been a serious problem previously for these modes, has been becoming distinct as a factor of degradation in displaying quality.
As one measure for improving color tones and viewing angle characteristics in the black state, disposing a birefringent optical compensation material between the liquid crystal layer and the polarizing plate has been tried for the LCD employing the IPS or FFS mode. For example, there is a disclosure describing that coloration in the white state or the half-tone state viewed at an oblique angle can be reduced by disposing birefringent media, having optical axes normal to each other, which can compensate increase and decrease in retardation of the liquid crystal layer when liquid crystal molecules in the layer are obliquely aligned, between the substrate and each of the polarizing plates (see Japanese Laid-Open Patent Publication, occasionally referred to as “JPA”, No. H9-80424). There have been also proposed a method of using an optical compensation film composed of styrene-base polymer or discotic liquid crystalline compound having negative intrinsic birefringence (see JPA Nos. H10-54982, H11-202323 and H9-292522); a method of using an optical compensation film obtained by combining a film having a positive birefringence and having an optical axis aligned in plane, and a film having a positive birefringence and having an optical axis aligned in the direction of the normal line of the film (see JPA No. H11-133408); a method of using a biaxial optical compensation film having a retardation of half wavelength (see JPA No. H11-305217); and a system of using a film having a negative retardation as a protective film of the polarizing plates, and providing on the surface thereof an optical compensation layer having a positive retardation (see JPA No. H10-307291).
In JPA No. 2005-265889, an IPS liquid crystal display device, provided with a compensation film composed of a stretched cellulose acylate film having an in-plane retardation Re and Nz of 1.5 to 7, and a rod-like liquid crystal coated and aligned thereon, is disclosed. The IPS liquid crystal display device successfully improves viewing angle characteristic, as one display quality, with a simple configuration.